This invention pertains to video signal processors, and in particular to an edge enhancement circuit that dynamically adjusts the amount of edge enhancement in accordance with alternatively the amount of automatic gain control applied to the video signal processor's input video signal or to the brightness level of the video signal.
A basic approach to improving the perceived sharpness of a displayed image corresponding to a viewed scene (a video signal), is to create and subsequently add an enhancement or peaking signal to the original video signal. The addition of such an enhancement signal makes the rise in a transition from a dark image region to a light image region, or alternatively the fall in a transition from a light image region to a dark image region, more steep. A circuit that performs this enhancement function is known as an edge enhancement circuit.
These enhancement signals are conventionally generated by increasing the amplitudes of the high frequency components of the video signal to steepen the positive and negative slopes of the original video signal. This steepening is variously performed by taking a first or second derivative of the original video signal, and appropriately adding or subtracting that derivative to the original video signal, or forming a peaking signal by subtracting from the original video signal a delayed video signal and appropriately adding or subtracting that peaking signal to the original video signal.
A problem in edge enhancement is that in a low light level situation, the high frequency components of an original video signal contain a disproportionate quantity of noise. Edge enhancement compensation boosts the resultant noise level to an unfavorable magnitude. An additional and related problem is that in a video processing circuit with automatic gain control, the darker viewed scenes undergo an automatic gain control amplification that boosts the noise in the resultant video signal. Thus, in a low light level video signal, and particularly in a low light video signal enhanced by automatic gain control; edge enhancement, while sharpening the transition areas of a displayed image, at the same time increases the unwanted and sometimes objectionable noise in the low light level scenes.
For each viewed scene, there is a preferred combination of the amount of edge enhancement for the brightness of the scene and the magnitude of the automatic gain control amplification applied to the video signal.
No prior edge enhancement circuit dynamically and adaptively adjusts the amount of edge enhancement.
No prior art edge enhancement circuit dynamically adjusts the amount of edge enhancement in response to the brightness of the scene or the magnitude of the automatic gain control amplification applied to the video signal.
No prior art edge enhancement circuit precisely adjusts the amount of edge enhancement as a predetermined function of the brightness of the viewed scene and the amount of automatic gain control amplification applied to the video signal to achieve a preferred amount of edge enhancement for a given amount of scene brightness and automatic gain control amplification.